1. Technical Field
The present invention relates to eyepiece variable focal length optics for use in combination with objective optics as in astronomical telescopes, spotting scopes, binoculars, etc.
2. Related Art
Many proposals have heretofore been made in the art of eyepiece variable focal length optics of the type contemplated by the invention. For instance, U.S. Pat. No. 5,311,355 proposes an afocal variable focal length optics which is a combination of an eyepiece variable focal length optical system with an objective optical system and an erecting optical system. Unexamined Published Japanese Patent Laid-Open Nos. 134617/1987 and U.S. Pat. No. 5,734,509 propose independent eyepiece variable focal length optics which do not include an objective optical system or an erecting optical system. The eyepiece variable focal length system, taken alone and excluding the objective and erecting optical systems, is a so-called OE-ZOOM system consisting of the following three groups in order from the object side, a moving first lens group of negative power, a moving second lens group of positive power, arranged to be opposed to the first lens group with a field stop interposed therebetween, and a fixed third lens group of positive power. The first and second lens groups are moved in opposite directions not only to achieve variable focal length but also to compensate for the position of the virtual image. These conventional eyepiece variable focal length optics mentioned above can achieve variable power ratios of only about 2.
Eyepiece optics are generally required to have adequate eye relief and offer a satisfactorily large apparent visual field, but if this need is met, the optics are prone to become bulky. This tendency becomes noticeable if the variable power ratio approaches 3.
An object of the present invention is to provide eyepiece variable focal length optics that achieves a variable power ratio of about 3 and which yet is compact and can offer adequately large apparent visual field and long eye relief.
The stated object can be attained by eyepiece variable focal length optics, which comprise in order from objective optics with which it is used in combination, a negative first lens group, a positive second lens group, a positive third lens group and a positive fourth lens group, the first, second and third lens groups moving independently of each other along the optical axis during variable focal length but the fourth lens group being stationary, said eyepiece variable focal length optics satisfying the following condition (1):
0.4 less than {L23(max)xe2x88x92L23}/f(h)xe2x80x83xe2x80x83(1) 
where
L23 (max): a maximum value of the axial air gap between the second and third lens groups during variable focal length;
L23: the axial air gap between the second and third lens groups at the longest focal length state;
f(h): the shortest focal length of the overall eyepiece variable focal length optics.
Preferably, the eyepiece variable focal length optics further satisfies the following condition (1xe2x80x2):
0.65 less than {L23(max)xe2x88x92L23}/f(h).xe2x80x83xe2x80x83(1xe2x80x2) 
When the overall eyepiece variable focal length optics is set at the shortest focal length state, a telescope using the eyepiece variable focal length optics in combination with the objective optics provides the highest magnification, assuming that the objective optics has a fixed focal length. Of course, the objective optics may be constructed as a variable focal length objective optics to further increase the highest magnification.
In a specific embodiment, the respective lens groups are preferably moved as follows during variable focal length from the longest focal length side to the shortest focal length side: the first lens group moves monotonically (simply) toward the objective optics, the second lens group moves monotonically (simply) toward the eye, and the third lens group moves toward the eye in such a way that the distance from the second lens group is first increased and then decreased.
Preferably, the eyepiece variable focal length optics according the invention further satisfies the following condition (2):
7.0 less than L12/f(h)xe2x80x83xe2x80x83(2) 
where
L12: the axial air gap between the first and second lens groups at the shortest focal length state.
More preferably, the eyepiece variable focal length optics according to the invention further satisfies the following conditions (3) and (4):
2.0 less than f4/f23 less than 4.0xe2x80x83xe2x80x83(3) 
0.3 less than f23/f2 less than 0.6xe2x80x83xe2x80x83(4) 
where
f4: the focal length of the fourth lens group;
f23: the composite focal length of the second and third lens groups at the longest focal length state;
f2: the focal length of the second lens group.
More preferably, the eyepiece variable focal length optics according to the invention further satisfies the following condition (1xe2x80x2):
0.65 less than {L23(max)xe2x88x92L23}/f(h).xe2x80x83xe2x80x83(1xe2x80x2) 
The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2000-136623 (filed on May 10, 2000), which is expressly incorporated herein by reference in its entirety.